1. Field of the Invention
The present invention relates to proton gas sensors and to a method of detecting a gas using the proton gas sensor. The gases to be detected according to the present invention are chiefly hydrogen and carbon monoxide and include other gases such as silanes, arsines, sulfur dioxide and nitrogen oxides. More particularly, the invention relates to the detection of these gases in oxygen-containing atmospheres such as air.
2. Prior Art
Kiyoyama et al. disclose a proton conductor gas sensor in unexamined Japanese patent publication SHO No. 60-7358 or in "Proceedings of the International Meeting on Chemical Sensors," pp. 233-238, Kodansha, 1983 corresponding to the publication. The basic feature of this sensor is that it is operable at room temperature. Further various proton conductors are already known for use in such sensors. For example, the above publication mentions proton conductors such as zirconium phosphate (Zr(HPO.sub.4).sub.2.nH.sub.2 O), dodecamolybdophosphoric acid (H.sub.3 Mo.sub.12 PO.sub.40. 30H.sub.2 O), antimonic acid (Sb.sub.2 O.sub.5. nH.sub.2 O, wherein n is usually about 2) and Nafion (trademark of E. I. du Pont de Nemours for perfluorocarbon sulfonate). In addition, Iwahara et al. have made a report on the proton conductivity of SrCeO.sub.3 wherein the Ce element is partly substituted with Y, Yb or like element ("Proceedings of the International Meeting on Chemical Sensors," pp. 227-232, Kodansha, 1983).
The Kiyoyama et al. sensor disclosed in unexamined Japanese patent publication SHO No. 60-7358 comprises a proton conductor, and an active electrode of platinum or the like and an inactive electrode of silver, gold, graphite or the like which are connected to the conductor. The platinum electrode decomposes a gas such as hydrogen by an electrode reaction to produce protons. However, the inactive electrode of silver or the like exhibits no activity to decompose hydrogen, with the result that an electromotive force occurs between the two electrodes. Thus, hydrogen or like gas is detected in the absence of any reference gas. Kiyoyama et al. further show that not only hydrogen but also carbon monoxide is also detectable with proton conductors.
With the Kiyoyama et al. sensor, the electromotive force varies in proportion to the logarithm of the gas concentration. In detecting hydrogen at room temperature with the sensor, a tenfold increase in the gas concentration results in a variation of 140 mv in the electromotive force. Further with this sensor, the electromotive force is not zero even in clean air; the sensor exhibits an electromotive force of 400 to 500 mv in clean air. The electromotive force occurring in clean air is thought attributable to an interfacial potential resulting from the difference between the two electrodes.
We have conducted research to obtain an output in proportion to the gas concentration and to obtain outputs with zero-point compensation, i.e., outputs relative to the output in clean air which is corrected to zero, and found that if two electrodes are short-circuited, the short-circuit current measured provides an output which is zero in clean air on proportion to the gas concentration. The current results from the travel of protons from an active electrode to an inactive electrode. On the other hand, the aforementioned electromotive force is not due to the travel of protons but to a hybrid potential at the active electrode. The electromotive force is measured with the external impedance to be connected between the two electrodes made almost infinitely great. In the case of the short-circuit current, the two electrodes are short-circuited externally to permit protons to travel in the interior of the conductor and obtain the resulting current.
Nevertheless, the short-circuit current was found sensitive to humidity, especially to relative humidity (FIG. 6), whereas the electromotive force was low in humidity dependence.